RNA interference (RNAi) has been used to silence the expression of a target gene. RNAi is a sequence-specific post-transcriptional gene silencing mechanism triggered by double-stranded RNA (dsRNA). It causes degradation of mRNAs homologous in sequence to the dsRNA. The mediators of the degradation are 21-23-nucleotide small interfering RNAs (siRNAs) generated by cleavage of longer dsRNAs (including hairpin RNAs) by DICER, a ribonuclease III-like protein. Molecules of siRNA typically have 2-3-nucleotide 3′ overhanging ends resembling the RNAse III processing products of long dsRNAs that normally initiate RNAi. When introduced into a cell, they assemble an endonuclease complex (RNA-induced silencing complex), which then guides target mRNA cleavage. As a consequence of degradation of the targeted mRNA, cells with a specific phenotype of the suppression of the corresponding protein product are obtained (e.g., reduction of tumor size, metastasis, angiogenesis, and growth rates).
The small size of siRNAs, compared with traditional antisense molecules, prevents activation of the dsRNA-inducible interferon system present in mammalian cells. This helps avoid the nonspecific phenotypes normally produced by dsRNA larger than 30 base pairs in somatic cells. See, e.g., Elbashir et al., Methods 26:199-213 (2002); McManus and Sharp, Nature Reviews 3:737-747 (2002); Hannon, Nature 418:244-251 (2002); Brummelkamp et al., Science 296:550-553 (2002); Tuschl, Nature Biotechnology 20:446-448 (2002); U.S. Application US2002/0086356